1. Priority Claim
This application claims the benefit of European Patent Application No. 04014262.2, filed Jun. 17, 2004. The disclosure of the above application is incorporated in its entirely herein by reference.
2. Technical Field
This application is directed to a diversity system with identification and evaluation of antenna properties. In particular, this application is directed to a mobile broadcast reception system to be used for the reception of broadcast signals in a vehicle.
3. Related Art
Modem vehicles are being equipped with more broadcast reception equipment than merely FM radio. Thus, it is becoming increasingly important to ensure quality reception as well as flexibility of use of equipment mounted in or attached to a vehicle. For example, such equipment may include terrestrial broadcast televisions, including analog, digital, DAB receivers and the like. Because the frequency band and signals for various receivers may be different, different reception antennas may be required.
To achieve quality reception similar to reception achieved in a stationary home or work environment, diversity reception antennas may be employed in mobile broadcast reception systems. Diversity reception generally implies spatial diversity. Another method that may be used is cross-polarization diversity, which may address problems associated with restricted space in vehicles.
However, a disadvantage with current diversity as employed in mobile reception systems is time varying multi-path fading, with different multi-path intensity profiles. Multi-path fading may arise in wireless broadcast as a result of reflections from stationary and non-stationary objects and is manifested as a random amplitude and phase modulation. At a receiver, multiple copies of a signal are summed together in either a constructive or destructive manner. The destructive addition of the signals may create fading dips in the signal power. The exact phase relationship, and therefore the degree of cancellation, may vary from position to position, making it possible for an antenna at location “A” to experience severe destructive cancellation and an antenna at location “B” to experience constructive addition. The distances involved depend upon frequencies used for transmission and may be small.
Diversity techniques aim to improve reception performance by allowing more than one antenna to be used with a common receiver. These antennas may be spatially separated by an appropriate distance or have different polarizations. Thus, selecting the best antenna on a dynamic basis provides some operational advantage such as automatically and dynamically recovering the highest possible signal quality. For example, multi-path fading is especially an issue in orthogonal frequency division multiplexing (OFDM) as generally utilized in digital video broadcast (DVB). OFDM is a method of digital modulation in which a signal is split into narrowband channels at different frequencies. In some respects, OFDM is similar to conventional frequency-division multiplexing (FDM). The difference, however, lies in how the signals are modulated and demodulated. Priority is given to minimizing the interference, or crosstalk, among the symbols making up the data stream. In other words, less importance is placed on perfecting individual channels.
Thus, a typical multi-path fading environment may include a signal transmitted from a transmitter received by a receiver mounted in, for example, a vehicle. In this situation, the signal transmitted may be received directly by the receiver, as well as after having been reflected off various objects in the surrounding environment such as buildings and/or trees. These different signals received are not correlated. However, for many scattering environments, spatial diversity is an effective way to improve the performance of wireless radio systems. The signals (at least two) should be received by the diversity antennas and then switched between or combined in the receiver.
A standard diversity technique is maximum ratio combining in a receiver, which means that the signal is down-converted into the base band, demodulated and then combined to optimize the signal to noise ratio. Alternatively, in switched diversity, one or the other of at least two antennas is selected and one of the antennas remains selected until the received signal strength falls below some limit of acceptability. At this point, the other antenna is switched and this process is repeated.
For example, one system may include a space diversity television broadcast receiver in a vehicle that can detect whether an antenna is connected or not, and subsequently choose the best signal of the connected antennas. In such a system, the video signal only includes the signals from the actually connected antennas, which means that harsh noise may be effectively suppressed. The harsh noise would result from the inclusion of a lacking portion of the video or audio signal resulting from the antenna connector signal during a specific period when one of the antennas is not connected. In practice, an antenna connection detection portion is included in between the respective antenna and tuner, and an unoccupied antenna connector detecting portion outputs a signal to a signal selecting controller which also feeds back into the tuner. In this way it is ascertained that only the signals from connected antennas are compared and can be selected.
Systems such as described above have been limited to a particular frequency band and determining whether an antenna is connected or not. However, frequency diversity needs to be employed because sometimes the same program is broadcast in two different frequency bands. Because any two different frequencies may experience different multi-path fading, it would be useful to receive these two different frequencies. Therefore, a need exists for a diversity system with identification and evaluation of antenna properties and more particularly to a mobile broadcast reception system to be used for the reception of broadcast signals in a vehicle which, among other things, is not limited to a particular frequency band and determining whether an antenna is connected or not.